Process for preparing a 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol derivative

ABSTRACT

Intermediates of Formula (III)                    
     wherein Y 1  is acyloxy, wherein acyl is of the formula R—C(O)—, wherein R is alkyl of 1-6 carbon atoms, Y 2  is aralkyloxy, and Z is benzoyloxy or acyloxy, wherein acyl is as defined above. 
     Intermediates of formula (III) are useful in a novel process for preparing the mono-L-valine ester of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol (ganciclovir). The mono-L-valine ester of ganciclovir and its pharmaceutically acceptable salts are of value as antiviral agents with improved absorption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/127,380, filedJul. 31, 1998, U.S. Pat. No. 6,040,446, which is a continuation-in-partof application Ser. No. 08/779,540, filed Jan. 8, 1997, now abandoned,which is in turn a continuation-in-part of application Ser. No.08/592,079, filed Jan. 26, 1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a prodrugformulation of ganciclovir and its pharmaceutically acceptable salts.More specifically, the invention relates to a process for preparing theL-monovaline ester derived from2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol and itspharmaceutically acceptable salts. The invention also relates to novelintermediates useful in the above process and to a process for preparingthe intermediates.

2. Background Information

British Patent 1 523 865 describes antiviral purine derivatives with anacyclic chain in the 9-position. Among those derivatives2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-ethanol with the INNname acyclovir has been found to have good activity against herpesviruses such as herpes simplex.

U.S. Pat. No. 4,355,032 discloses the compound9-[(2-hydroxy-1-hydroxymethylethoxy)methyl]guanine or2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol with theINN name ganciclovir. Ganciclovir is highly efficacious against virusesof the herpes family, for example, against herpes simplex andcytomegalovirus.

British Patent Application GB 2 122 618 discloses derivatives of9-(2-hydroxyethoxymethyl)guanine of the generic formula:

wherein X represents an oxygen or sulfur atom, R¹ represents a hydroxyor an amino group, R₂ represents a hydrogen atom or a group of theformula CH₂OR³ _(a) and R³ and R³ _(a) may be the same or different,each represents an amino acid acyl radical and physiologicallyacceptable salts thereof. These compounds can be prepared by condensinga guanine derivative with a side chain intermediate in a strong polarsolvent such as dimethylformamide or hexamethylphosphoramide,advantageously in the presence of a base, or by thermal condensation inthe presence of a strong acid. These compounds are useful for thetreatment of viral infections and have high water solubility whichrenders them of value in the formulation of aqueous pharmaceuticalpreparations. While the generic formula in the British patentapplication includes compounds in which R² is the group —CH₂OR³ _(a),specific compounds of this group are not disclosed.

European Patent Application 0 375 329 A2 discloses prodrug compoundswith the following formula

wherein R and R¹ are independently selected from hydrogen and an aminoacid acyl residue providing at least one of R and R¹ represents an aminoacid acyl residue and B represents a group of the formulae

in which R² represents a C₁₋₆ straight chain, C₃₋₆ branched chain orC₃₋₆ cyclic alkoxy group, or a hydroxy or amino group or a hydrogen atomand the physiologically acceptable salts thereof. These prodrugcompounds are described as having advantageous bioavailability whenadministered by the oral route, resulting in high levels of the parentcompound in the body.

Example 3(b) of European Patent Application 0 375 329 discloses thepreparation of the bis(L-isoleucinate) ester of ganciclovir as a whitefoam. Example 4(b) discloses the preparation of the bis(glycinate) esterof ganciclovir as a white solid. Example 5(b) discloses the preparationof the bis(L-valinate) ester of ganciclovir as a solid. Example 6(b)discloses the preparation of the bis(L-alaninate) ester of gancicloviras a syrup containing 90% of the bis-ester and 10% of the monoester. Thebis-esters are prepared by reacting ganciclovir with an optionallyprotected amino acid or functional equivalent thereof; the reaction maybe carried out in a conventional manner, for example in a solvent suchas pyridine, dimethylformamide, etc., in the presence of a couplingagent such as 1,3-dicyclohexylcarbodiimide, optionally in the presenceof a catalytic base such as 4-dimethylaminopyridine. The describedbis-esters are non-crystalline materials which are difficult to processfor the manufacture of oral pharmaceutical dosage forms.

British Patent Application No. 8829571 is the priority patentapplication for European Patent Application 0 375 329 and U.S. Pat. No.5,043,339, and discloses amino acid esters of the compounds of theformula

(wherein R represents a hydroxy or amino group or a hydrogen atom) andthe physiologically acceptable salts thereof. Examples of preferredamino acids include aliphatic acids, e.g., containing up to 6 carbonatoms such as glycine, alanine, valine and isoleucine. The amino acidesters include both mono and diesters. The preparation of the diestersis identical to the preparation in European Patent Application 0 375329; however, this patent application as well as European PatentApplication 0 375 329 and U.S. Pat. No. 5,043,339 do not disclose thepreparation of monoesters, or any data suggesting their usefulness.

Leon Colla et al., J. Med. Chem. (1983) 26, 602-604 disclose severalwater-soluble ester derivatives of acyclovir and their salts as prodrugsof acyclovir. The authors indicate that acyclovir cannot be given as eyedrops or intramuscular injections because of its limited solubility inwater and have therefore synthesized derivatives of acyclovir which aremore water soluble than the parent compound. The authors disclose thehydrochloride salt of the glycyl ester, the hydrochloride salt of thealanyl ester, the hydrochloride salt of the beta-alanyl ester, thesodium salt of the succinyl ester, and the azidoacetate ester. Thealanyl esters were prepared by conventional esterification methods,including reacting acyclovir with the corresponding N-carboxy-protectedamino acid in pyridine, in the presence of 1,3-dicyclohexyl-carbodiimideand a catalytic amount of p-toluenesulfonic acid and subsequentcatalytic hydrogenation to give the alpha- and beta-alanyl esters astheir hydrochloride salts.

L. M. Beauchamp et al., Antiviral Chemistry & Chemotherapy (1992), 3(3),157-164 disclose eighteen amino acid esters of the antiherpetic drugacyclovir and their efficiencies as prodrugs of acyclovir, evaluated inrats by measuring the urinary recovery of acyclovir. Ten prodrugsproduced greater amounts of the parent drug in the urine than acycloviritself: the glycyl, D,L-alanyl, L-alanyl, L-2-aminobutyrate, D,L-valyl,L-valyl, DL-isoleucyl, L-isoleucyl, L-methionyl, and L-prolyl ester.According to the authors the L-valyl ester of acyclovir was the bestprodrug of the esters investigated. These esters were prepared bymethods similar to those employed by Colla et al.

European Patent Application 0 308 065 A2 discloses the valine andisoleucine esters of acyclovir, preferably in the L-form, as showing alarge increase in absorption from the gut after oral administration,when compared with other esters and acyclovir. The amino acid esters areprepared by conventional esterification methods, including reactingacyclovir with an N-carboxy-protected amino acid or an acid halide oracid anhydride of the amino acid, in a solvent such as pyridine ordimethylformamide, optionally in the presence of a catalytic base. Theamino acid esters of acyclovir may also be prepared by condensing aguanine derivative with an amino acid side-chain intermediate in amanner analogous to that disclosed in British Patent Application GB 2122 618, discussed above.

PCT Patent Application WO 94/29311 discloses a process for thepreparation of amino acid esters of a nucleoside analogue, includingacyclovir and ganciclovir. This process comprises reacting a nucleosideanalogue having an esterifiable hydroxy group in its linear or cyclicether moiety, with a 2-oxa-4-aza-cycloalkane-1,3-dione of the formula

wherein R¹ may represent hydrogen, C₁₋₄ alkyl or alkenyl group or otheramino acid side chains, and R² may represent hydrogen or a group COOR³where R³ is a benzyl, t-butyl, fluorenylmethyl or an optionally halosubstituted linear or branched C₁₋₈ alkyl group. Preferred R¹ groupsinclude hydrogen, methyl, isopropyl and isobutyl, yielding respectivelythe glycine, alanine, valine and isoleucine esters of acyclovir organciclovir. Examples 1-3 of PCT Patent Application WO 94/29311discloses only the condensation of acyclovir with the valine-substituted2-oxa-4-aza-cycloalkane-1,3-dione (Z-valine-N-carboxyanhydride) byconventional procedures. While the amino acid esters of the PCTapplication include both the acyclovir and ganciclovir (DHPG) esters,the application does not disclose how to prepare the ganciclovir esters,much less the mono-esters of ganciclovir.

The L-monovaline ester derived from2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol and itspharmaceutically acceptable salts are potent antiviral agents and aredescribed in European Patent Application 0 694 547 A. These compoundshave been found to have improved oral absorption and low toxicity. Thispatent application also discloses certain processes for preparing theseesters, different from those described herein.

The present invention relates to an improved process and novelintermediates whereby an acid addition salt of a mono-hydroxy protectedganciclovir is formed as a novel intermediate, which reduces impuritiesin the mono-valine ester end-product, compared to known intermediates.This also eliminates the costly and time consuming purification stepsand allows the use of starting materials of lower purity, which, inturn, reduces overall production costs.

SUMMARY OF THE INVENTION

In a first aspect, this invention provides a process for preparing thecompound of the formula (I):

and pharmaceutically acceptable salts thereof, which compound is namedhereinafter2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propyl-L-valinateor mono-L-valine ganciclovir.

This process involves the condensation of a silylated guanine compoundwith a substituted glycerol derivative, optionally followed by formationof an acid addition salt of a mono-hydroxy protected ganciclovir as anintermediate; esterification of this product with an L-valine derivativeand the removal of any protecting groups forms the prodrug of Formula(I). Optionally, the process can also include the formation of salts ofthe prodrug of Formula (I), the conversion of an acid addition salt ofthe prodrug of Formula (I) into a non-salt form, the optical resolutionof a prodrug of Formula (I) or the preparation of the prodrugs ofFormula (I) in crystalline form. Details of the process are describedbelow.

In a second aspect, this invention provides compounds of Formula (IV)and Formula (V) which are useful intermediates for preparingmono-L-valine ganciclovir and its pharmaceutically acceptable salts.

The compounds of Formula (IV) are:

wherein Z¹ is hydrogen or an amino-protecting group, Y¹ is halo, loweracyloxy or aralkyloxy, and Y² is lower acyloxy.

The compounds of Formula (V) are:

wherein Z¹ is hydrogen or an amino-protecting group, Y² is halo, loweracyloxy or aralkyloxy. Compounds of Formula (V) may optionally beconverted into an acid addition salt; preferred is the hydrochloridesalt.

A third aspect of this invention is a process for preparing the novelintermediates of Formula (IV) and (V).

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Unless otherwise stated, the following terms used in the specificationand claims have the meanings given below:

“BOC” means t-butoxycarbonyl.

“CBZ” means carbobenzyloxy (benzyloxycarbonyl).

“FMOC” means N-(9-fluorenylmethoxycarbonyl).

“DHPG” means 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine.

“Alkyl” means a straight or branched saturated hydrocarbon radicalhaving from 1-12 or one to the number of carbon atoms designated. Forexample, C₁₋₇ alkyl is alkyl having at least one but no more than sevencarbon atoms, e.g. methyl, ethyl, i-propyl, n-propyl, n-butyl, n-pentyl,n-heptyl and the like.

“Lower alkyl” means an alkyl of one to six carbon atoms.

“Aryl” means an organic radical derived from an aromatic hydrocarbon bythe removal of one hydrogen atom. Preferred aryl radicals are aromaticcarbocyclic radicals having a single ring (e.g., phenyl) or twocondensed rings (e.g. naphthyl).

“Aralkyl” means an alkyl group in which an alkyl hydrogen atom isreplaced by an above-defined aryl group.

“Acyl” means an organic radical of the formula R—C(O)—, derived from anorganic acid by the removal of the hydroxyl group; R is alkyl or aryl of1-12 carbon atoms; e.g., CH₃CO— is the acyl radical of acetic acid(CH₃COOH), or acetyl. Another examples for acyl is propionyl. Benzoyl isthe acyl radical of benzoic acid (C₆H₆COOH), etc.

“Lower acyl” refers to “acyl” when it represents “alkanoyl” which is theorganic radical RCO— in which R is an alkyl group of 1-6 carbon atoms;preferred lower acyl groups are the acetyl and the propionyl radicals.

“Lower alkyloxy”, “(lower alkyl)amino”, “di(lower alkyl)amino”, “(loweralkanoyl)amino”, and similar terms mean alkoxy, alkylamino,dialkylamino, alkanoylamino, etc. in which the or each alkyl radical isa “lower alkyl” as described above.

“Halogen” or “halo” means fluorine, chlorine, bromine, or iodine.

“Trityl” means the triphenylmethyl radical (PH)₃C—.

“Mesyl” means the methanesulfonyl radical CH₃SO₂—.

“Tosyl” means the p-toluenesulfonyl radical CH₃C₆H₅SO₂—.

“Aprotic (nonpolar) solvent” means organic solvents such as diethylether, ligroin, pentane, hexane, cyclohexane, heptane, octane, benzene,toluene, dioxane, tetrahydrofuran, carbon tetrachloride, and the like.

“Phase transfer catalyst” means a catalyst which alters the rate oftransfer of water-soluble reactant across the interface to the organicphase. Suitable catalysts are, e.g., tetrabutylammonium chloride,tetrabutylammonium bromide, tetrabutylphosphonium chloride,tetrabutylphosphonium bromide, and N-2-ethylhexyl-4-dimethylaminopyridinium bromide.

“Derivative” of a compound means a compound obtainable from the originalcompound by a simple chemical process.

“Activated derivative” of a compound means a reactive form of theoriginal compound which renders the compound active in a desiredchemical reaction, in which the original compound is only moderatelyreactive or non-reactive. Activation is achieved by formation of aderivative or a chemical grouping within the molecule with a higher freeenergy content than that of the original compound, which renders theactivated form more susceptible to react with another reagent. In thecontext of the present invention activation of the carboxy group is ofparticular importance and corresponding activating agents or groupingswhich activate the carboxy group are described in more detail below. Anexample of an activated derivative of L-valine is the compound ofFormula (VI):

wherein P² is an amino-protecting group and A is a carboxy-activatinggroup, for example, halo, a lower acyloxy group, a carbodiimide groupsuch as 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC), anisobutyrate group, and the like.

Of particular interest for the present invention is an amino acidanhydride which is an activated form of an amino acid which renders theamino acid (especially L-valine) susceptible to esterification. Aminoacid anhydrides are included in the compounds of Formula (VI), above.Especially useful for the present invention are the cyclic amino acidanhydrides of L-valine, described in PCT Patent Application WO 94/29311,such as 2-oxa-4-aza-5-isopropyl-cycloalkane-1,3-dione of Formula (VIa):

in which P² is an amino protecting group. Other examples of the cyclicamino acid anhydrides are protected amino acid N-carboxyanhydrides(NCA's) described in more detail below.

“Protecting group” means a chemical group that (a) preserves a reactivegroup from participating in an undesirable chemical reaction; and (b)can be easily removed after protection of the reactive group is nolonger required. For example, the benzyl group is a protecting group fora primary hydroxyl function.

“Amino-protecting group” means a protecting group that preserves areactive amino group that otherwise would be modified by certainchemical reactions. The definition includes the formyl group or loweralkanoyl groups with 2 to 4 carbon atoms, in particular the acetyl orpropionyl group, the trityl or substituted trityl groups such as themonomethoxytrityl group, dimethoxytrityl groups such as the4,4′-dimethoxytrityl, the trichloroacetyl group, the trifluoroacetylgroup, the silyl group, the phthalyl group, and theN-(9-fluorenylmethoxycarbonyl) or “FMOC” group, the allyloxycarbonylgroup, or other protecting groups derived from halocarbonates such as(C₆-C₁₂)aryl lower alkyl carbonates (such as the N-benzyloxycarbonylgroup derived from benzylchlorocarbonate), or derived from biphenylalkylhalocarbonates, or tertiary alkyl halocarbonates such as tertiarybutylhalocarbonates, in particular tertiary butylchlorocarbonate, ordi(lower)alkyldicarbonates, in particular di(t-butyl)dicarbonate, andthe triphenylmethyl halides such as triphenylmethyl chloride, andtrifluoroacetic anhydride.

“Hydroxy-protecting group” means a protecting group that preserves ahydroxy group that otherwise would be modified by certain chemicalreactions. In the context of the present invention, thehydroxy-protecting group can be an ether- or ester-forming group thatcan be removed easily after completion of all other reaction steps, suchas a lower acyl group (e.g., the acetyl or propionyl group), or anaralkyl group (e.g., the benzyl group, optionally substituted at thephenyl ring).

“Silylation catalyst” as used herein refers to catalysts that promotethe silylation of guanine, for example ammonium sulfate,p-toluenesulfonic acid, trifluoromethane sulfonic acid,trimethylsilyltrifluoromethane sulfonate, bistrimethylsilyl sulfonate,sulfuric acid, potassium butylsulfonate, ammonium perchlorate, sodiumperchlorate, sodium borofluoride or tin tetrachloride.

“Silylating agent” as used herein refers to a compound capable ofsilylating guanine. A preferred silylating agent is hexamethyldisilazane(which will give a compound of Formula (II)) in which R⁵, R⁶, and R⁷ areall methyl). However, many other silylating agents are known in the art.For example, guanine may be reacted with a trialkylsilyl halide offormula SiR⁵R⁶R⁷X, in which R⁵, R⁶, and R⁷ are independently lower alkyland X is chloro or bromo, such as trimethylsilyl chloride,tert-butyldimethylsilyl chloride, and the like, preferably in thepresence of about 1-2 molar equivalents of a base.

The silylated/persilylated compound of Formula (II) is represented asfollows:

Formula (II) represents guanine protected by one, two, or three silylgroups, or a mixture thereof, where Z¹, Z², and Z³ are independentlyhydrogen or a silyl group of formula SiR⁵R⁶R⁷, provided that at leastone of Z¹, Z², and Z³ must be a silyl group, in which R⁵, R⁶, and R⁷ areindependently lower alkyl. It should be noted that Formula (II) as drawnrepresents a mixture of N-7 and N-9 isomers (as a tautomeric mixture).

Optionally, Formula (II) may represent the case where Z¹ is anamino-protecting group other than silyl as defined in the specification,and Z² and Z³ are independently hydrogen or silyl.

“Leaving group” means a labile group that is replaced in a chemicalreaction by another group. Examples of leaving groups are halogen, theoptionally substituted benzyloxy group, the mesyloxy group, the tosyloxygroup or the acyloxy group.

All the activating and protecting agents employed in the preparation ofthe compound of Formula (I) must meet the following qualifications: (1)their introduction should proceed quantitatively and withoutracemization of the L-valine component; (2) the protecting group presentduring the desired reaction should be stable to the reaction conditionsto be employed; and (3) the group must be readily removable underconditions in which the ester bond is stable and under whichracemization of the L-valine component of the ester does not occur.

The process of the invention may also include the optical resolution ofa prodrug of Formula (I). Terminology relating to the stereochemistryand optical resolution of these compounds is described in EuropeanPatent Application 0 694 547 A, incorporated herein by reference.

“Optional” or “optionally” means that a described event or circumstancemay or may not occur, and that the description includes instances wheresaid event or circumstance occurs and instances in which it does not.For example, “optionally substituted phenyl” means that the phenyl mayor may not be substituted and that the description includes bothunsubstituted phenyl and phenyl wherein there is substitution;“optionally followed by converting the free base to the acid additionsalt” means that said conversion may or may not be carried out in orderfor the process described to fall within the invention, and theinvention includes those processes wherein the free base is converted tothe acid addition salt and those processes in which it is not.

“Pharmaceutically acceptable” means that which is useful in preparing apharmaceutical composition that is generally safe and non-toxic andincludes that which is acceptable for veterinary use as well as humanpharmaceutical use.

“Pharmaceutically acceptable salts” means salts which possess thedesired pharmacological activity and which are neither biologically norotherwise undesirable. Such salts include acid addition salts formedwith inorganic acids such as hydrochloric acid, hydrobromic acid,sulfuric acid, nitric acid, phosphoric acid, and the like; or withorganic acids such as acetic acid, propionic acid, hexanoic acid,heptanoic acid, cyclopentane-propionic acid, glycolic acid, pyruvicacid, lactic acid, malonic acid, succinic acid, malic acid, maleic acid,fumaric acid, tartaric acid, citric acid, benzoic acid,o-(4-hydroxybenzoyl)benzoic acid, cinnamic acid, mandelic acid,methanesulfonic acid, ethanesulfonic acid, 1,2-ethane-disulfonic acid,2-hydroxyethanesulfonic acid, benzenesulfonic acid,p-chlorobenzenesulfonic acid, 2-naphthalenesulfonic acid,p-toluenesulfonic acid, camphorsulfonic acid,4-methyl-bicyclo[2.2.2]oct-2-ene-1-carboxylic acid, glucoheptonic acid,4,4′-methylene-bis-(3-hydroxy-2-naphthoic)acid, 3-phenylpropionic acid,trimethylacetic acid, tertiary butylacetic acid, lauryl sulfuric acid,gluconic acid, glutamic acid, hydroxynaphthoic acids, salicylic acid,stearic acid, muconic acid, and the like.

Preferred pharmaceutically acceptable salts are those formed withhydrochloric acid, sulfuric acid, phosphoric acid, acetic acid,methanesulfonic acid, ethanesulfonic acid, 1,2-ethanedisulfonic acid,2-hydroxyethanesulfonic acid, benzene-sulfonic acid,p-chlorobenzenesulfonic acid, 2-naphthalenesulfonic acid,p-toluenesulfonic acid and camphorsulfonic acid.

Synthetic Reaction Parameters

Unless specified to the contrary, the reactions described herein takeplace at atmospheric pressure within a temperature range from 5° C. to170° C. (preferably from 10° C. to 50° C.; most preferably at “room” or“ambient” temperature, i.e., about 20° to 30° C.). However, there areclearly some reactions where the temperature range used in the chemicalreaction will be above or below these temperature ranges. Further,unless otherwise specified, the reaction times and conditions areintended to be approximate, e.g., taking place at about atmosphericpressure within a temperature range of about 5° C. to about 100° C.(preferably from about 10° C. to about 50° C.; most preferably about20°-30° C.) over a period of about 1 to about 100 hours (preferablyabout 5 to 60 hours). Parameters given in the Examples are intended tobe specific, not approximate. Isolation and purification of thecompounds and intermediates described herein can be effected, ifdesired, by any suitable separation or purification procedure such as,for example, filtration, extraction, crystallization, columnchromatography, thin-layer chromatography or thick-layer chromatography,or a combination of these procedures. Specific illustrations of suitableseparation and isolation procedures can be had by reference to theexamples hereinbelow. However, other equivalent separation or isolationprocedures can, of course, also be used.

Presently Preferred Embodiments

While the broadest definition of this invention is set forth in theSummary of the Invention as a process for preparing the compound ofFormula (I) and its pharmaceutically acceptable salts, the (R,S) mixtureand certain salts are preferred.

The following acids are preferred to form pharmaceutically acceptablesalts with the compound of Formula (I): hydrochloric, sulfuric,phoshoric, acetic, methanesulfonic, ethanesulfonic,1,2-ethanedisulfonic, 2-hydroxyethanesulfonic, benzenesulfonic,p-chlorobenzenesulfonic, 2-naphthalenesulfonic, p-toluenesulfonic andcamphorsulfonic acid. Most preferred are strong inorganic acids such ashydrochloric, sulfuric or phosphoric acid.

The most preferred compounds are2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propyl-L-valinatehydrochloride and acetate. These compounds can be prepared ascrystalline materials and therefore can be easily manufactured intostable oral formulations.

In any of the last step processes described herein, a reference toFormulae (I), (II), (III), (IV), (V), (VI), (VIa) or (VII) refers tosuch Formulae wherein Z¹, Z², Z³, and P², A, Y¹, Y², Z and X are asdefined in their broadest definitions set forth in the Summary of theInvention, with the processes applying particularly to the presentlypreferred embodiments.

Details of the Synthetic Processes

The process of the present invention is depicted in the ReactionSequence shown below:

wherein Z¹, Z², and Z³ are independently hydrogen or a silyl group ofthe formula —SiR⁵R⁶R⁷, provided that at least one of Z¹, Z², and Z³ mustbe a silyl group; optionally, Z¹ may be an amino-protecting groupselected from the group consisting of lower alkanoyl, optionallysubstituted trityl, trifluoroacetyl and N-(9-fluorenylmethoxycarbonyl.

The compounds of Formula (III) are glycerol derivatives wherein Y¹ andY² independently are halo, lower acyloxy, or aralkyloxy, or one of Y¹ orY² is a valyloxy group, and Z is a leaving group selected from loweracyloxy, benzoyloxy, halo, mesyloxy or tosyloxy, and the like. Ingeneral, Y¹ and Y² of the glycerol derivative need to be chosen in sucha way as to permit the obtention of the mono-L-valine ester of Formula(I). One of Y¹ or Y² can be an amino-protected L-valyloxy group, or agroup convertible to the L-valyloxy group.

The guanine compound of Formula (II) is condensed with a 2-substitutedglycerol of the Formula (III) to yield a compound of Formula (IV), whichis a 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl) methoxy-1,3-propanediol(ganciclovir) intermediate with protection at both hydroxy functions (orprotection at one hydroxy function when one of Y¹ or Y² is a valyloxygroup) and optionally at the 2-amino group of guanine.

When both hydroxy functions are protected, the compound of Formula (IV)is then de-protected at one of the hydroxy functions to provide themono-protected ganciclovir intermediate of Formula (V).

Optionally, a pharmaceutically acceptable acid addition salt of thecompound of Formula (V) may be prepared. Preferred is the hydrochlorideacid addition salt of Formula (V).

Compounds of Formula (V) may be esterified with an activated derivativeof L-valine of Formula (VI) or (VIa) to provide the compounds ofFormulae (VII), optionally followed by removal of amino- and/orhydroxy-protecting groups to form a compound of Formula (I).

If the valyloxy group is introduced in Step (a) using a glycerolderivative where one of Y¹ and Y² is an amino-protected L-valyloxy groupor a group convertible to the L-valyloxy group, the resulting compoundof Formula (IV) or (V) is converted directly to a compound of Formula(I) by removal of the hydroxy- and amino-protecting groups.

Compounds of Formula (I) can optionally be converted into apharmaceutically acceptable salt thereof. The process can also includethe conversion of an acid addition salt of the prodrug of Formula (I)into a non-salt form, the optical resolution of a compound of Formula(I) or the preparation of the compound of Formula (I) in crystallineform.

The present invention is an improved process for the preparation ofmono-L-valine ganciclovir, in which the formation of the intermediate ofFormula (V) provides distinct advantages over the previously knownprocedures. This novel intermediate may optionally be converted into anacid addition salt of a mono-hydroxy protected ganciclovir, whichprovides for a substantial reduction in several impurities which may bepresent in the commercially available guanine used to prepare theguanine derivative of Formula (II). These impurities may otherwise becarried through to the desired end-product.

In addition, the starting material for the preparation of some of theglycerol reagents of Formula (III) can be contaminated by certainimpurities. These impurities are not removed during synthesis of theglycerol reagent, and when the reagent is reacted with guanine in thecondensation reaction, it will give rise to the corresponding isomericganciclovir impurities. For example, the starting material for theglycerol reagent of Formula (III), wherein Y¹ is benzyloxy and Y² and Zare propionyloxy, can be the compound 1-benzyloxy-3-chloro-2-propanol.This starting material can contain 2-chloro-3-benzyloxypropanol or2-benzyloxy-3-chloropropanol. Either of these impurities will give thecorresponding impurity in the glycerol reagent and, in the ensuingcondensation reaction with guanine, the impurity will carry through asan isomeric impurity of the ganciclovir intermediate.

Furthermore, the reaction of guanine with the glycerol reagent ofFormula (III) gives a mixture of product isomers: the desired9-substituted guanine (the 9-isomer) and a small amount of the undesired7-substituted guanine (the 7-isomer). If the glycerol reagent containsthe impurities discussed above, then the corresponding impurities ofganciclovir will also be present. None of these impurities can beremoved easily from the desired 9-isomer.

The present invention provides for the optional generation of an acidaddition salt of the compounds of Formula (IV) or (V), which allows forthe isolation of the end-product essentially free of the 7-isomer andwith levels of the impurities reduced by at least 50%. This acidaddition salt intermediate can be prepared directly from the guaninereaction mixture which contains the dihydroxy-protected compound ofFormula (IV). Alternatively, and in a preferred embodiment, the compoundof Formula (IV) can be first deprotected at one of the hydroxy groups toprovide the mono-hydroxy protected ganciclovir of Formula (V), fromwhich intermediate the acid addition salt may then be prepared. Also,from the compound of Formula (IV), one can first prepare theintermediate with protection at both hydroxy moieties and at the 2-aminomoiety of the guanine group with, for example, an acyl anhydride. Thisprocedure is advantageous because the fully protected intermediate canbe crystallized free of the undesired 7-isomer. These fully protectedcompounds are novel intermediates and are those compounds of the generalFormula (IV), wherein Z¹ is an amino-protecting group which is loweracyl, Y¹ is halo, lower acyloxy or aralkyloxy, and Y² is lower acyloxy,so that the acyl groups of Z¹ and Y² are the same. A preferred,fully-protected intermediate is dipropionyl-monobenzyl ganciclovir ordiacetyl-monobenzyl ganciclovir.

In general, the process for producing the compounds of Formula (I) mayor may not involve protection of the amino group in the 2-position ofthe guanine base. These protecting groups may be removed prior to theformation of the salt intermediate of Formula (V), after theesterification step or in the last deprotection step. For the case whenthe ganciclovir intermediates have a protected 2-amino group theprotecting group may be removed by conventional procedures. For example,if the amino-protecting group is a lower alkanoyl group, basicconditions (pH between 8 to 11) are employed to remove the protectinggroup. For example, a 2-N-acetyl-ganciclovir intermediate is treatedwith an alkaline reagent such as ammonium hydroxide, sodium or potassiumcarbonate or sodium or potassium hydroxide until the removal of theacetyl group is complete. In general, this reaction will be conducted inthe presence of a suitable solvent such as a lower alkanol. Preferablythe starting material is dissolved in methanol and a stoichiometricexcess of ammonium hydroxide is added. The reaction temperature is keptbetween 0° to 50° C., preferably at room temperature. After the reactionis complete (which can be determined by TLC), another solvent may beadded to facilitate isolation of the de-protected product, such as ethylether which leads to precipitation of the de-acylated product which canbe filtered off and isolated using conventional separation methods.

In general, when carrying out a process of this invention, those amino,hydroxy or carboxylic groups which are not to participate in thesynthesis reaction must be protected until (1) either de-protectionyields the final product; or (2) the presence of the unprotected groupin the ensuing reaction steps leading to the final product would notmodify the intended sequence of reactions. An example for meetingrequirement (1) is the benzyloxycarbonyl group in the preparation of thefinal product of this invention, which protects the amino group of thevaline function of ganciclovir until it is removed in the de-protectionstep. An example for meeting requirement (2) is the acetyl group, or thetrityl or monomethoxytrityl group protecting the amino group of theguanine ring system of ganciclovir, as the unprotected amino group doesnot interfere with the esterification (Step c).

In general, the qualification of potential blocking agents that renderthem suitable for use in the preparation of the compound of Formula (I)include:

(1) Their introduction should proceed quantitatively and smoothlywithout L-valine racemization;

(2) The blocked intermediate must be stable to conditions of thereactions employed until removal of the protecting group is required;

(3) The blocking group must be susceptible of being readily removedunder conditions which do not change the chemical nature of theremainder of the molecule or result in racemization of the L-valinecomponent.

where Z¹, Z² and Z³ are independently hydrogen or a silyl protectinggroup of the formula R⁵R⁶R⁷Si, in which R⁵, R⁶, and R⁷ are independentlylower alkyl, provided that at least one of Z¹, Z² and Z³ is a silylgroup.

Preparation of Silylated/Persilylated Guanine of Formula (II)

The trialkylsilyl halides of formula R⁵R⁶R⁷SiX (where X is chloro orbromo) or hexamethyldisilazane are commercially available.

As illustrated in Reaction Scheme B, guanine is silylated to give thecorresponding protected compound of Formula (II).

The protection of guanine is well known in the art (see, for example“Synthesis of 9-substituted Guanines. A Review” by F. P. Clausen and J.J. Christensen, Org. Prep. Proced. Int., 25(4), pp 375-401 (1993)).Guanine may, for example, be protected using acyl groups, for exampleacetyl, or by silyl groups. Traditionally, when silyl groups areemployed for protection, guanine is silylated in such a manner that allactive protons present in guanine are replaced by a silyl group beforeproceeding with the desired reaction, i.e. guanine is protected as thetrisilyl derivative. However, it has been found that, althoughtrisilylation of guanine followed by the condensation of Step (a) givesthe desired product in good yield, and indeed is preferred, it is notessential that guanine be trisilylated for the condensation carried outin Step (a) to be essentially specific for the preparation of compound(IV). Conventionally, guanine as a slurry is reacted with a silylatingagent, for example hexamethyldisilazane, at reflux until all suspendedmaterial goes into solution, which signals the complete formation of thetrisilyl derivative. This reaction can take up to 48 hours or more. Ithas been found that refluxing for much less time, for example as littleas 2 hours, then reacting the slurry thus produced with a compound ofFormula (III) as described in Step (a), gives good yields of desiredproduct. This result is clearly advantageous, since less expense isinvolved in a shortened reaction time, and smaller amounts of silylatingreagent are used. Although the composition of a compound of Formula (II)produced by reacting guanine with hexamethyldisilazane for a shortenedperiod of time is not yet known with any certainty, it is believed to bemainly a monosilyl derivative., probably mixed with some disilyl andtrisilyl guanine.

In one preferred method, guanine is reacted with about 3-10 molarequivalents of a silylating agent, preferably with hexamethyldisilazane(i.e. to give a compound of Formula (II) where R⁵, R⁶, and R⁷ are allmethyl) , in the presence of an silylation catalyst, preferably ammoniumsulfate, trifluoromethanesulfonic acid, trimethylsilyltrifluoromethanesulfonate, or bistrimethylsilyl sulfonate, most preferablytrifluoromethanesulfonic acid (about 0.01 to 0.1 molar equivalents). Themixture is heated to reflux over a period of about 5-24 hours,preferably about 16 hours. When the reaction is substantially complete,excess silylating agent is removed under reduced pressure, and theresultant solution of the protected guanine product of Formula (II) isused in the next step without further purification.

Alternatively, guanine is reacted with a silylating agent, preferablyhexamethyldisilazane, in the presence of a silylating catalyst,preferably trifluoromethanesulfonic acid, as described in the precedingparagraph, but for a period of about 1-8 hours, preferably 2-4 hours.Optionally, excess silylating agent is removed under reduced pressure,and the resultant mixture of the protected guanine product of Formula(II) is used in the next step without further purification.

Alternatively, guanine may be reacted with 1-5 molar equivalents of atrialkylsilyl halide of formula SiR⁵R⁶R⁷X, in which R⁵, R⁶, and R⁷ areindependently lower alkyl and X is chloro or bromo, such astrimethylsilyl chloride, tert-butyldimethylsilyl chloride, and the like,in the presence of about 1-5 molar equivalents of a base.

It should be noted that ammonium sulfate, trifluoromethanesulfonic acid,trimethylsilyltrifluoromethane sulfonate, or bistrimethylsilyl sulfonatework well as silylation catalysts in the silylation of guanine describedabove. However, use of trifluoromethanesulfonic acid is preferredbecause it is much less expensive than trimethylsilyltrifluoromethanesulfonate or bistrimethylsilyl sulfonate.

Starting Materials

All starting materials employed to make the compound of Formula (I) areknown, such as guanine and the protecting andcarboxylic-group-activating reagents.

The glycerol derivatives of Formula (III) which are used in thecondensation reaction with guanine or a protected guanine compound aredescribed in European Patent Applications 0 694 547 A and 0 187 297.European Patent Application 0 187 297 also describes certain methods forpreparing the glycerol derivatives of Formula (III). A preferred methodfor preparing the glycerol derivatives is described below in the section“Preparation of Glycerol Derivatives”.

A preferred guanine derivative is the silylated/persilylated guanine.Preferred glycerol derivatives are1-benzyloxy-3-propionyloxy-2-(propionyloxy)methoxypropane,1-benzyloxy-3-acetyloxy-2-(acetyloxy)methoxypropane, or1-benzyloxy-3-benzyloxy-2-(acetyloxy)methoxypropane.

Prior to carrying out Step (c) (esterification step), the amino group ofthe L-valine derivative must be protected to avoid its interference withthe esterification by undesirable amide formation. The variousamino-protected L-valine derivatives useful in this invention, such asN-benzyloxy-carbonyl-L-valine, BOC-L-valine and EMOC-L-valine,N-formyl-L-valine and N-benzyloxycarbonyl-N-carboxy-L-valine anhydride,are all commercially available (SNPE Inc., Princeton, N.J., AldrichChemical Co., Milwaukee, Wis., and Sigma Chemical Co., St. Louis, Mo.),or are described in the literature, such as N-allyloxycarbonyl-L-valine.Cyclic amino-protected L-valine derivatives are also described in theliterature, as noted above. Of particular interest for the presentinvention is the benzyloxycarbonyl valine-substituted2-oxa-4-aza-cyclo-alkane-1,3-dione (Z-valine-N-carboxyanhydride, orZ-Valine-NCA), which is also commercially available (SNPE Inc.,Princeton, N.J.) . Alternatively, the protecting step may be carried outby conventional methods.

Preparation of Glycerol Derivatives of Formula (III)

The glycerol derivatives useful in this invention can be prepared fromknown starting materials. For example, the compounds of Formula (III)wherein Y¹ is lower aralkyloxy or halo, Y² is lower acyloxy or halo, andZ is lower acyloxy, can be prepared as described below. This reaction isexemplified by the preparation of the compounds wherein Y¹ is benzyloxy,Y² is propionyloxy and Z is propionyloxy, i.e.,1-benzyloxy-3-propionyloxy-2-(propionyloxy) methoxypropane.

Epichlorohydrin is reacted with benzyl alcohol in the presence oftetrabutylammonium bisulfate in aqueous sodium hydroxide, at roomtemperature. The product of this reaction, benzyl glycidyl ether, isisolated by conventional means and is then added slowly to a suspensionof lithium chloride in tetrahydrofuran and acetic acid, at 40°-70° C.,preferably below 60° C. The reaction mixture is allowed to cool to roomtemperature, and stirred for 2-10 hours, preferably 3-6 hours. Theproduct is isolated by extraction, washed and dried to provide1-benzyloxy-3-chloro-2-propanol. To this product is then addedmethoxymethyl propionate, which is prepared by adding propionicanhydride to dimethoxymethane in the presence of an ion exchange resin,e.g., Amberlyst 15, maintaining the temperature between 40°-60° C.,preferably between 40°-50° C. during the addition. The reaction mixtureis aged and cooled, then filtered, washed and distilled. This product,methoxymethyl propionate, is reacted with1-benzyloxy-3-chloro-2-propanol in an aprotic solvent, e.g., hexanes, inthe presence of p-toluenesulfonic acid hydrate at reflux. Distillationand washing affords the product1-benzyloxy-3-chloro-2-(propionyloxy)methoxypropane. Finally, to preparethe compounds of Formula (III),1-benzyloxy-3-chloro-2-(propionyloxy)methoxypropane, is refluxed with analkali metal alkanoate, e.g., sodium propionate, in an aprotic solvent,e.g., toluene, after which a phase transfer catalyst such astetrabutylphosphonium chloride is added. The reaction mixture is stirredat 90° C. to reflux temperature for 1-3 days, preferably 2 days, atwhich time more tetrabutylphosphonium chloride and solvent may be added,if required. The mixture is heated to reflux and the distillate removed,then stirred at 90° C. to reflux temperature for 3-16 hours, preferably5-10 hours, then cooled to ambient temperature. The mixture is thenwashed with water and brine, and the organic phase is separated andconcentrated to yield 1-benzyloxy-3-propionyloxy-2-(propionyloxy)methoxypropane. In an analogous manner, other glycerol derivatives of Formula(III) may be prepared.

Further nonlimiting examples of phase transfer catalysts or agents thatmay be employed in the preparation of compounds of Formula (III) arereviewed by C. M. Starks, C. L. Liotta, and M. Halpern in“Phase-Transfer Catalysis”, Chapman & Hall, New York, 1994, which isincorporated herein in its entirety by reference.

Preparation of Activated derivative of L-valine

Prior to carrying out Step (c) (esterification step), L-valine must alsobe activated. At least 1 equivalent of the protected amino acid and 1equivalent of a suitable coupling agent or dehydrating agent, forexample 1,3-dicyclohexylcarbodiimide or salts of such diimides withbasic groups should be employed from the start. Other carbodiimides suchas N,N′-carbonyldiimidazole may also be used. Further useful dehydratingagents are trifluoroacetic anhydride, mixed anhydrides, acid chlorides,1-benzo-triazolyloxy-tris-(dimethylamino)phosphoniumhexafluorophosphate, benzotriazol-1-yl-oxy-trispyrrolidinophosphoniumhexafluorophosphate, 1-hydroxybenzotriazole,1-hydroxy-4-azabenzotriazole, 1-hydroxy-7-azabenzotriazole,N-ethyl-N′-(3-(dimethylamino) propyl)carbodiimide hydrochloride,3-hydroxy-3,4-dihydro-4-oxo-1,2,3-benzotriazine,O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate,O-(7-azabenzotriazol-1-yl) -1,1,3,3-tetramethyluroniumhexafluorophosphate,O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate,O-(1H-benzotriazol-1-yl)-1,1,3,3-bis(tetramethylene) uroniumhexafluorophosphate orO-(7-azabenzotriazol-1-yl)-1,1,3,3-bis-(tetramethylene)uroniumhexafluorophosphate.

A description of these coupling agents by L. A. Carpino can be found inJ. Am. Chem. Soc. 1993, 115, p. 4397-4398.

Also useful for this purpose are urethane-protected amino acid N-carboxyanhydrides (UNCA's) which are an activated form of an amino acid; thesehave been described by William D. Fuller et al., J. Am. Chem. Soc. 1990,112, 7414-7416, which is incorporated herein by reference. Otherprotected amino acid N-carboxy anhydrides are described in PCT PatentApplication WO 94/29311, discussed above. In summary, any other reagentthat produces an anhydride or another activated derivative of theprotected amino acid under mild conditions can be used as the couplingagent.

The amino-protected amino acid is dissolved in an inert solvent such asa halogenated lower alkane, preferably dichloromethane, under an inertatmosphere, for example nitrogen, and the coupling agent is added(preferably 1,3-dicyclohexylcarbodiimide). The reaction mixture isstirred at temperatures between 0° and 50° C., preferably at about roomtemperature. The reaction mixture is filtered and the reaction product(the anhydride of the protected amino acid) isolated. The resultingproduct is dissolved in a dry inert solvent such as dry dichloromethaneand placed under nitrogen.

Preparation of Mono-L-valine Ganciclovir

Step (a): Condensation

The reaction conditions for the condensation of guanine with the 2-aminogroup optionally protected, are described in European Patent Application0 187 297. In this condensation reaction, guanine is reacted with aglycerol derivative of Formula (III) in an aprotic hydrocarbon solvent(such as benzene, toluene or xylenes) or dimethylformamide with ahexa-lower alkyl(di)silazane, for example, hexamethyldisilazane,hexaethyldisilazane, or the like, and a catalyst at temperatures between30° C. and reflux temperature. The catalyst is a Lewis acid salt such astrialkyl silyl salt (e.g., the sulfate), or a trifluoroalkyl sulfonicacid, a chlorosilane, or ammonium sulfate and pyridine. For a moredetailed disclosure of the reaction conditions for condensation (Step(a)) see the disclosure of European Patent Application 0 187 297 whichis incorporated herein by reference. The resulting compound is aganciclovir derivative with protected hydroxy groups and with anoptionally protected 2-amino group.

For example, a ganciclovir intermediate of Formula (IV), where Y¹ isbenzyloxy and Y² is lower acyloxy, can be prepared by condensingpersilyl guanine with a glycerol derivative of Formula (III) where Y¹ isbenzyloxy and Y² and Z are lower acyloxy. Typically, persilyl guanine istreated with a large excess of a glycerol derivative of Formula (III) inthe presence of a catalytic amount of a Lewis acid salt, preferablytrifluoromethane sulfonic acid at 60°-150° C. preferably 110°-130° C.for 3-24 hours, preferably 6-8 hours. The mixture is cooled, dilutedwith an aprotic nonpolar solvent, preferably toluene, and then water isadded carefully. The product can optionally be isolated by filtration.

Step (b): Hydrolysis

The protected ganciclovir derivative of Formula (IV) from Step (a) ispartially de-protected to provide ganciclovir with the 2-amino groupoptionally in protected form and one protected primary hydroxylfunction. Preferably, the primary hydroxyl function is protected with abenzyl group. Suitable amino-protecting groups are lower alkanoyl groupswith 2 to 4 carbon atoms, in particular the acetyl or propionyl group.Other suitable amino-protecting groups are the trityl or substitutedtrityl groups such as the monomethoxytrityl group, and the4,4′-dimethoxytrityl group.

As noted above, the acid addition salt of the compound of Formula (V),can be prepared directly from the product of Step (a), which is thedihydroxy-protected compound of Formula (IV), by de-protecting one ofthe hydroxy groups with concomitant preparation of the salt.Alternatively, the compound of Formula (IV) can first be deprotected atone of the hydroxy groups to provide the mono-hydroxy protectedganciclovir of Formula (V), from which the acid addition salt is thenprepared. Also, the compound of Formula (IV), the intermediate withprotection at both hydroxy groups as well as at the 2-amino guaninegroup can be prepared, with, for example, an acyl anhydride. Forexample, the dipropionyl monobenzyl ganciclovir intermediate is preparedfrom the propionyl monobenzyl ganciclovir intermediate of Formula (IV)by reaction with propionic anhydride/dimethylaminopyridine, in, forexample, toluene. As discussed above, the ganciclovir intermediate withprotection at both hydroxy groups and at the 2-amino guanine group, suchas dipropionyl monobenzyl ganciclovir, is a preferred intermediatebecause it can be isolated substantially free of the undesired 7-isomerof guanine.

When one of Y¹ and Y² is aralkyloxy, or when both Y¹ and Y² arearalkyloxy, for example, benzyloxy, then deprotection occurs byhydrogenolysis under conventional hydrogenation conditions; when one ofthe groups Y¹ or Y² is acyloxy or halo, said group is selectivelyremoved by basic hydrolysis.

Transfer hydrogenation conditions can also be employed: a palladiumcatalyst such as palladium hydroxide is used in a suitable solvent suchas cyclohexene. A cosolvent such as methanol, ethanol or isopropanol maybe necessary for better solubility of the adduct.

Hydrogenolysis is preferably carried out by dissolving the protectedganciclovir in a solvent system under conventional hydrogenationconditions at 5-100 psi (0.3-7 atm), preferably 10-40 psi (0.7-2.8 atm)hydrogen, in the presence of a catalyst such as a palladium compound, inparticular palladium hydroxide on carbon (Pearlman's catalyst), at about20°-60° C., preferably 20°-35° C., until completion of the reaction.Other suitable hydrogenation catalysts include hydrogenation catalystsin general such as Pd, Pd on carbon and homogeneous hydrogenationcatalysts. The solvent system includes a lower alkanol such as methanolor ethanol. Generally the reaction will be carried out at temperaturesbetween room temperature and the reflux temperature of the solventsystem, for example in refluxing ethanol under a hydrogen atmosphere andunder exclusion of air. The reaction vessel is preferably swept withnitrogen prior to charging it with hydrogen. The catalyst will berecovered by filtration. The filtrate can be reduced in volume byevaporation of excess solvent. The resulting crude reaction mixturegenerally includes unchanged starting material and 2-amino-protectedganciclovir with one aliphatic hydroxy group protected as the majorproducts. The separation of these two products is usually performed byisolation procedures known in the art, often by chromatographic methods,preferably on silica gel, followed by elution with appropriate eluentssuch as mixtures of a lower alkanol with a halogenated lower alkane(preferably ethanol and dichloromethane) to give 2-amino-protectedganciclovir with one aliphatic hydroxy group protected. This ganciclovirintermediate can then be isolated as the salt compound of Formula (V) byconventional methods, using, for example, hydrogen chloride and asolvent such as methanol.

The hydrolysis reaction to remove an acyl hydroxy-protecting group ispreferably carried out by treating the protected ganciclovir under basichydrolysis conditions. The hydrolysis medium may include a lower alkylalcohol such as methanol or ethanol, toluene, and aqueous sodiumhydroxide. Generally the reaction will be carried out at temperaturesbetween room temperature and the reflux temperature of the solventsystem. Again, this ganciclovir intermediate can be isolated as the saltcompound of Formula (V) as described above.

For example, the product obtained in Step (a) can be partiallydeprotected by removing the lower acyl group (of Y¹ or Y²) with base.After the reaction described in Step (a) is complete and the reactionmixture has been cooled and diluted with, preferably, methanol, aqueoussodium hydroxide is added. The mixture is heated to 40°-90° C.,preferably 60°-80° C., until the reaction is complete. The reactionmixture is then carefully acidified with hydrochloric acid. The productis collected as the hydrochloride salt by filtration, then washed anddried.

Step (c): Esterification

In this step an activated derivative of amino-protected L-valine of theFormula (VI) or (VIa) is esterified with the mono-hydroxy protectedganciclovir salt derivative of Formula (V) obtained in Step (b).Suitable amino-protecting groups for the L-valine derivative are theN-benzyloxycarbonyl group, the phthalyl group, the tertiarybutyloxycarbonyl group and the N-(9-fluorenylmethoxycarbonyl) or “FMOC”group.

A suspension of the product of Step (b)(the compound of Formula (V) inan aprotic solvent (preferably dimethylformamide) containing an organicbase (preferably TEA) is added to an approximately equivalent amount ofthe activated L-valine derivative in an aprotic solvent (preferablydimethylformamide). The activated L-valine derivative is preferablyZ-valine-N-carboxyanhydride or L-valine anhydride. The reaction mixtureis stirred at 0°-40° C. preferably at 4°-10 ° C., for 1-5 hours. Thereaction mixture is diluted with water, preferably toluene and water.The precipitate is collected by filtration, washed and dried at ambienttemperature.

Step (d): Final De-protection to Give the Product of Formula (I)

The valine protecting groups of the product of Step (c), the hydroxyprotecting group Y² and optionally any 2-amino guanine protecting groupsare removed by de-protection reactions, preferably in an acidic mediumor solvent, most preferably by hydrogenation. De-protection under acidicconditions is preferred, as this will ensure that the amino groupliberated in the de-protection reaction will be protonated; that is,that the base of Formula (I) as it is formed in the de-protectionreaction will be captured by an at least stoichiometric amount of acidpresent. Isolating the compound of Formula (I) as an acid addition saltwill protect the desired stereo configuration of the compound of Formula(I). Therefore, those examples given below that show the de-protectionstep also show the concomitant salt formation step.

The de-protection reaction is carried out by dissolving the product ofthe esterification step (c) in an inert solvent, preferably in an acidicsolvent, using a hydrogenation catalyst such as palladium on carbon, orpalladium hydroxide on carbon (Pearlman's catalyst), using elevatedhydrogen pressure between 1 and 2000 psi (0.1-140 atm), preferably 20 to200 psi (1.4-14 atm). The completion of the reaction can be monitoredusing conventional TLC analysis. The hydrogenolysis is continued untilthe conversion is complete, if required with addition of furtherhydrogenation catalyst. The catalyst is removed and washed. The combinedfiltrates from filtration and the washings are concentrated andlyophilized to isolate the ganciclovir L-valine ester. The purificationof the product and the isolation of a crystalline ester is carried outby recrystallization or other purification techniques such as liquidchromatographic techniques.

The hydrogenolysis may be slow due to the presence of impurities(catalyst poisons) in the starting material. It has been found to beadvantageous to treat the starting material prior to hydrogenolysis inmethanol with filtering aids such as catalytic Filtrol®, Solka Floc® andactivated carbon such as ADP carbon. This effectively removes mostcatalyst poisons.

If the tertiary butyloxycarbonyl group is being used as amino-protectinggroup, its removal is effected with acid such as HCl and isopropanol asa solvent or with trifluoroacetic acid neat.

Alternatively, if the esterification step has been carried out with atrityl or substituted trityl-protected ganciclovir derivative, suchprotecting groups can be removed by treatment with an aqueous alkanoicacid or trifluoroacetic or hydrochloric acid at temperatures between−20° C. and 100° C., for example, aqueous acetic acid.

Preparation of Salts

One of ordinary skill in the art will also recognize that the compoundof Formula (I) may be prepared either as an acid addition salt or as thecorresponding free base. If prepared as an acid addition salt, thecompound can be converted to the free base by treatment with a suitablebase such as ammonium hydroxide solution, sodium hydroxide, potassiumhydroxide or the like. However, it is important to point out that thefree base of Formula (I) is more difficult to characterize than its acidaddition salts. When converting the free base to an acid addition salt,the compound is reacted with a suitable organic or inorganic acid(described earlier). These reactions are effected by treatment with anat least stoichiometric amount of an appropriate acid (in case of thepreparation of an acid addition salt) or base (in case of liberation ofthe free compound of Formula (I)). In the salt-forming step of thisinvention typically, the free base is dissolved in a polar solvent suchas water or a lower alkanol (preferably isopropanol) or mixturesthereof, and the acid is added in the required amount in water or inlower alkanol. The reaction temperature is usually kept at about 0° to50° C., preferably at about room temperature. The corresponding saltprecipitates spontaneously or can be brought out of the solution by theaddition of a less polar solvent such as ether or hexane, removal of thesolvent by evaporation or under vacuum, or by cooling the solution.

Isolation of Stereoisomers and the Manufacture of Crystalline2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propyl-L-valinate

From the Formula (I) it is apparent that the compound of the inventionhas one asymmetric carbon atom (chiral center) in the propyl chain, inaddition to the asymmetric carbon atom in L-valine. Therefore, twodiastereomeric forms exist, the (R)— and (S)— form as determined by therules of Cahn et al. Suitable methods for the separation of thediastereomers are described in European Patent Application 0 694 547 A,incorporated herein by reference.

The compounds of Formula (I) may also be prepared in crystalline form,which has many well-known advantages over the non-crystalline form.Suitable methods for the preparation of the compounds of the inventionin crystalline form are also described in European Patent Application 0694 547 A, incorporated herein by reference.

The following preparations and examples are given to enable thoseskilled in the art to more clearly understand and to practice thepresent invention. They should not be considered as limiting the scopeof the invention, but merely as being illustrative and representativethereof.

EXAMPLE 1 1A. Preparation of2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propyl-propionate

Trifluoromethane sulfonic acid (0.5 ml) was added to guanine (25 g) andthe mixture was briefly agitated. Hexamethyldisilazane (HMDS) (125 ml)was added and the mixture was heated to reflux until solution wasachieved. The solution was vacuumed distilled to remove excess HMDS. Theresidue was cooled and more trifluoromethane sulfonic acid (0.4 ml) wasadded, followed by1-benzyloxy-3-propionyloxy-2-(propionyloxy)methoxypropane (70 g). Themixture was heated at 110°-130° C. for several hours until little or noguanine was detected by HPLC. The mixture was cooled and diluted withtoluene (150 ml) and methanol (21 ml). Water (20 ml) was added carefullyand the mixture was then cooled. Propionyl monobenzyl ganciclovir (29 g)was collected by filtration, washed with toluene and water and dried.

1B. Preparation of2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propyl-acetate

Trifluoromethane sulfonic acid (0.5 ml) was added to guanine (25 g) andthe mixture was briefly agitated. Hexamethyldisilazane (HMDS) (125 ml)was added and the mixture was heated to reflux until solution wasachieved. The solution was vacuumed distilled to remove excess HMDS. Theresidue was cooled and more trifluoromethane sulfonic acid (0.4 ml) wasadded followed by 1-benzyloxy-3-acetyloxy-2-(acetyloxy)methoxypropane(65 g). The mixture was heated at 110°-130° C. for several hours untillittle or no guanine was detected by HPLC. The mixture was cooled anddiluted with toluene (75 ml). Water (25 ml) was added carefully and themixture was then cooled. Acetyl monobenzyl ganciclovir (38 g) wascollected by filtration, washed with toluene and water and dried.

1C. Preparation of2-(2-acetylamino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-dibenzyloxy-propane

In a completely analogous manner to that described in Examples 1A and1B,2-(2-acetylamino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-dibenzyloxy-propanewas prepared using 1-benzyloxy-3-benzyloxy-2-acetyloxymethoxypropane asthe glycerol reagent and 2-N-acetyl-guanine.

EXAMPLE 2

Preparation of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propanol hydrochloride

2A. The preparation of the mono-protected ganciclovir intermediate as asalt (the compound of Formula V) was prepared directly from the productof Example 1A.

Monobenzyl ganciclovir hydrochloride was isolated as the product of theprocedure described in Example 1A by using the following modification:After the reaction was complete and was cooled and diluted with methanol(250 ml), NaOH (23 g) was added. The mixture was heated with goodagitation. When hydrolysis was judged complete (HPLC, TLC), the mixturewas cooled and conc. hydrochloric acid (45.2 g) added. The mixture wasfiltered and the filtrate diluted with ethyl acetate (240 ml). Themixture was cooled and the product collected, washed with ethyl acetateand dried to yield 30.0 g.

2B. Similarly, monobenzyl ganciclovir hydrochloride was prepared fromacetylmonobenzyl ganciclovir (the product of Example 1B) by heating amixture of sodium hydroxide (10.0 g), methanol (150 ml) andacetylmonobenzyl ganciclovir (49 g) until the reaction was complete. Thesolution was acidified with hydrochloric acid (31 g) and the mixture wasfiltered. The filtrate was diluted with ethyl acetate (750 ml) andcooled. The product was collected by filtration, washed with ethylacetate and dried to yield 47 g.

EXAMPLE 3

Preparation of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propanol hydrochloride

3A. The preparation of the mono-protected ganciclovir intermediate as asalt (the compound of Formula V) was also prepared via the non-saltintermediate(2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propanol,or monobenzyl ganciclovir.

Monobenzyl ganciclovir was isolated as the product of the proceduredescribed in Example 1A by using the following modification: After thereaction was complete and was cooled and diluted with toluene (25 ml), asolution of NaOH (25 g) in water (125 ml) was added. The mixture washeated with good agitation. When hydrolysis was judged complete (HPLC,TLC), the lower aqueous layer was slowly added to a hot mixture ofacetone (125 ml), acetic acid (25 g) and water (25 ml) with goodagitation. The mixture was cooled and monobenzyl ganciclovir isolated byfiltration, washed with aqueous acetone and dried.

Next, monobenzyl ganciclovir hydrochloride was prepared from monobenzylganciclovir (17 g) by mixing with conc. hydrochloric acid (5 ml) andmethanol (80 ml) and warming until all solid dissolved. The solution wasdiluted with ethyl acetate (160 ml) and cooled. The product wascollected by filtration, washed with ethyl acetate and dried to yield18.1 g.

While a preferred solvent for preparing monobenzyl ganciclovirhydrochloride from monobenzyl ganciclovir is methanol, other solventscan be used in an analogous manner. Such other solvents includeisopropanol, ethanol and butanol.

3B. Alternatively, monobenzyl ganciclovir and monobenzyl ganciclovirhydrochloride were prepared from the product of Example 1B.

Monobenzyl ganciclovir was isolated as the product of the proceduredescribed in Example 1B by using the following modification: After thereaction was complete and was cooled and diluted with toluene (25 ml), asolution of NaOH (25 g) in water (125m) was added. The mixture washeated with good agitation. When hydrolysis was judged complete (HPLC,TLC), the lower aqueous layer was slowly added to a hot mixture ofacetone (125 ml), acetic acid (25 g) and water (25 ml) with goodagitation. The mixture was cooled and monobenzyl ganciclovir isolated byfiltration, washed with aqueous acetone and dried to yield 41 g.

Monobenzyl ganciclovir HCl was then prepared from monobenzyl ganciclovirin a manner similar to that described in Example 3A, above.

3C. Alternatively, monobenzyl ganciclovir and monobenzyl ganciclovirhydrochloride were prepared from the product of Example 1C. In thisexample, the product of Example 1C is a 2-amino protected dibenzylganciclovir intermediate of Formula (IV).

First, N-acetyl-dibenzyl-ganciclovir was converted toN-acetyl-monobenzyl-ganciclovir. N-acetyl-dibenzyl-ganciclovir (14.5 Kg)was charged to a 200 liter glass reactor along with 60.1 Kg methanol,and 900 g of Pearlman's catalyst. This mixture was placed under ahydrogen atmosphere and heated to 40° C. for 11 hours. The catalyst wasremoved by filtration through a Solka Floc® cake. This cake was washedwith 60 Kg of methanol. Methanol (60 kg) was distilled from the solutionof N-acetyl-dibenzyl-ganciclovir and N-acetyl-monobenzyl-ganciclovir.Water (113 kg) was added to this concentrated methanol solution. Thismixture was cooled to 5° C. overnight. The N-acetyl-dibenzyl-ganciclovirwas then removed by filtration and washed with 140 L of (6:4)methanol/water. The methanol/water solutions were combined andmethanol/water was distilled under vacuum to a jacket temperature of115° C., 27 ins of Hg (ca. 685 torr), and a pot temperature of 44° C.,until 260 Kg of methanol/water had distilled. The resulting aqueouslayer was extracted 3 times with 100 kg each of dichloromethane (eachdichloromethane extraction contained 3.75 L ethanol.) Thedichloromethane layers were combined and the dichloromethane/ethanol wasremoved by atmospheric distillation to a pot temperature of 40° C.Acetone (7.3 L) was added to the pot residue and the pot was heated to50° C. with agitation. This heterogeneous mixture was cooled to 5° C.overnight. The solid was filtered out and washed with 15 L (−5° C. to−10° C.) acetone. The solid was dried in a vacuum oven (˜50° C., 25 insof Hg (ca. 635 torr), nitrogen sweep) for 24 hours. Isolated yield ofN-acetyl monobenzyl-ganciclovir: 29% (3.425 Kg). HPLC: 91.7%N-acetyl-monobenzyl ganciclovir, 2.3% monobenzyl ganciclovir, 0.3%N-acetyl-ganciclovir.

Ammonolysis of N-acetyl-monobenzyl ganciclovir tomonobenzyl-ganciclovir: To 103 g N-acetyl-monobenzyl-ganciclovir wasadded 500 ml methanol and 100 ml 30% NH₄OH in water. The reaction wascomplete by TLC in about 22 hours. The methanol was evaporated from theheterogeneous mixture to a temperature of 40° C., at 28 ins of Hg (ca.710 torr). The aqueous solution was cooled to room temperature, and thenfiltered. The solid was washed with 500 ml water and dried in a vacuumoven (˜50° C., 25 ins of Hg (ca 635 torr), nitrogen sweep) overnight.Weight: 94.1 g (7179-94). HPLC: 95.5% monobenzyl ganciclovir.

Monobenzyl ganciclovir HCl was then prepared from monobenzyl ganciclovirin a manner similar to that described in Example 3A, above.

EXAMPLE 4

Preparation of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propanol hydrochloride

4A. The preparation of the mono-protected ganciclovir intermediate as asalt (the compound of Formula V) was also prepared via a 2-aminoprotected intermediate2-(2-propionyl-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propylpropionate.

Dipropionylmonobenzyl ganciclovir was isolated as the product of theprocedure described in Example 1A by using the following modification:After the reaction was complete and was cooled a solution of4-dimethylaminopyridine (1.6 g) in propionic anhydride (31 g) was addedand the mixture was heated until the acylation was complete (HPLC, TLC).Water (8.5 ml) was added and the hot mixture was extracted with hexane(160 ml) or a hexane (160 ml)/toluene (80 ml) mixture. The lower layerof the mixture was separated and diluted with toluene (150 ml). The hotsolution was washed with water (1×25 ml, 1×75 ml), diluted with ethylacetate (15 ml), and again washed with water (75 ml). The organic layerwas cooled and stirred. The product was collected by filtration, washedwith toluene and dried to yield 43 g.

Monobenzyl ganciclovir hydrochloride was prepared fromdipropionylmonobenzyl ganciclovir by heating a mixture of sodiumhydroxide (20.0 g), methanol (400 ml) and dipropionyl-monobenzylganciclovir (112 g) until the reaction was complete. The solution wasacidified with hydrochloric acid (73.5 g) and the mixture was filtered.The filtrate was diluted with ethyl acetate (800 ml) and cooled. Theproduct was collected by filtration, washed with ethyl acetate and driedto yield 76.7 g monobenzyl ganciclovir hydrochloride.

4B. Monobenzyl ganciclovir was also prepared from dipropionylmonobenzylganciclovir as follows: A mixture of sodium hydroxide (7 g), water (80ml) and dipropionylmonobenzyl ganciclovir (22.9 g) was heated until thereaction was complete. The mixture was added to a mixture of acetic acid(10 g) and water (20 ml) and was then cooled. The product was collectedby filtration, washed with water and dried to yield 17.1 g.

EXAMPLE 5

Preparation of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-benzyloxy-1-propyl-N-(benzyloxycarbonyl)-L-valinate

5A. N-CBZ-monovalinate-monobenzyl-ganciclovir was prepared frommonobenzyl ganciclovir by adding a solution ofCBZ-L-valine-N-carboxyanhydride (2.0 g) in dimethylformamide (0 ml) to amixture of triethylamine (0.2 g), dimethylformamide (2 ml), andmonobenzyl ganciclovir (2.0 g). The mixture was then diluted with moretriethylamine (0.2 g), toluene (2.4 ml) and water (8 ml) and wasvigorously stirred to initiate crystallization. More water (8 ml) wasadded and the mixture was cooled. The product was collected byfiltration, washed with water and dried, to yield 3.1 g.

5B. Alternatively, N-CBZ-monovalinate-monobenzyl-ganciclovir wasprepared from monobenzyl ganciclovir by adding a solution ofCBZ-L-valine anhydride in dimethylformamide (50 ml) to a mixture of4-dimethylaminopyridine(3.8 g), dimethylformamide (50 ml), andmonobenzyl ganciclovir (47.0 g). The anhydride was prepared by addingdicyclohexylcarbodiimide (36.0 g) to a stirred mixture of CBZ-L-valine(97.2 g) and ethyl acetate (280 ml). The mixture was stirred overnight,was filtered and the cake washed with ethyl acetate (150 ml). Thefiltrate was stripped and the residue dissolved in dimethylformamide andused as described above. Upon reaction completion, the mixture wasdiluted with triethylamine (20 g), toluene (50 ml) and water 200 ml) andwas vigorously stirred to initiate crystallization. More water (200 ml)was added and the mixture was cooled. The product was collected byfiltration, washed with water and dried, to yield 87.4 g.

5C. N-CBZ-monovalinate-monobenzyl-ganciclovir was prepared frommonobenzyl ganciclovir hydrochloride as follows:

To a mechanically stirred suspension of O-monobenzyl-ganciclovir HCl(25.0 g, 66.8 mmoles) in dimethylformamide (22 ml) at 4-70° C. under anatmosphere of nitrogen, was added triethylamine (7.4 g, 87 mmoles) atsuch a rate that the temperature of the slurry did not exceed 8° C. Oncethe addition was finished, the slurry was stirred at 4-6° C. while asolution of Z-Valine-NCA (24.0 g, 86 mmoles) in dimethyl-formamide (23ml) was added dropwise. After the addition was finished, the ice bathwas removed and the mixture was allowed to come to room temperature(23-25° C., approximately 30-45 min). Assay of the mixture by TLC(80:10:8 CH₃CN:CH₃COOH:H₂O) showed the reaction to be complete afterthis period of time. Successive addition to the mixture of triethylamine(2.2 g, 21.7 mmoles) toluene (17.5 ml) and water (20 ml) at 23-25° C.was followed by heating of the mixture to 40-46° C. The mixture wastreated dropwise with additional water (80 ml) and then slowly cooled to23-25° C. over a period of 2 hours. To the moderately vigorously stirredmixture was added water (100 ml) over a period of 10-15 minutes. Thesolid so formed was allowed to stir for a period of 10-15 minutes andthen collected by filtration. The filtercake was washed with 2 portionsof water (50 ml each) and air dried for 3 hours. Residual toluene wasremoved in vacuo at 35-40° C. Yield: 39.3 g (ca. 100%).

5D. N-CBZ-monovalinate-monobenzyl-ganciclovir can also be preparedadvantageously in superior purity from monobenzyl ganciclovirhydrochloride as follows: CBZ-valine-NCA (1.15 equivalents) is dissolvedin ethyl acetate and added to a slurry of monobenzyl ganciclovir (1.0equivalent) in the presence of 4-dimethylaminopyridine (3% by weight) indimethylformamide (DMF) and ethyl acetate at 23°-27° C. After thereaction mixture has been stirred for about 3 hours the mixture isanalyzed by HPLC for progress of the reaction. Stirring of the reactionmixture is continued until the reaction is judged essentially complete.Water is added to quench the reaction and ethyl acetate is added todilute the mixture. The organic phase is separated and the aqueous phaseis again extracted with ethyl acetate. The combined ethyl acetatesolution is washed twice with water, treated with activated carbon suchas PWA carbon at 35°-40° C. and then filtered and azeotropically driedand concentrated to a premarked volume. Hexane is slowly added at 89° C.and the resulting mixture is slowly cooled to 25° C. to crystallize theproduct. The mother liquor is removed by decantation and the product iswashed twice with an ethyl acetate/hexane (4/3) solution and once withhexane. The ethyl acetate/hexane and hexane washes are removed bydecantation. The pure product is isolated by filtration and dried.

EXAMPLE 6

Preparation of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propyl-L-valinate hydrochloride

Ganciclovir-L-valinate hydrochloride was prepared fromN-CBZ-monovalinate-monobenzyl-ganciclovir as follows: A solution of thestarting material (14.2 g) in methanol (100 ml) and conc. hydrochloricacid (2.7 g) was hydrogenated over palladium hydroxide on carbon(Pearlman's catalyst) (2.7 g). When the reaction was complete themixture was filtered and the filtrate was concentrated under vacuum to alow volume. Water (9 ml) was added and the solution again stripped toremove residual methanol. Isopropanol (35 ml) was added and the mixturewas stirred vigorously to initiate crystallization. More isopropanol (55ml) was added and the mixture was stirred and cooled. The product wascollected by filtration, washed with isopropanol and dried to yield 8.0g; MS: 355 (MH)⁺.

EXAMPLE 7

Preparation of1-Benzyloxy-3-propionyloxy-2-(propionyl-oxy)methoxypropane, a compoundof Formula (III)

7A. Preparation of Methoxymethyl Propionate

To 710 ml of dimethoxymethane in a 3-liter 3-neck round bottom flask wasadded 14.0 g of Amberlyst 15. The mixture was heated to reflux and 950ml of propionic anhydride was added as rapidly as possible (exothermicreaction) while the flask was cooled in an ice bath. The reactiontemperature was allowed to rise to 50-55° C. The mixture was stirred for5 minutes after the addition was complete and was then filtered directlyonto 14.0 g of anhydrous potassium carbonate. The filtrate wasfractionally distilled through an efficient column to separate residualdimethoxymethane, methyl propionate and the product (methoxymethylpropionate). Yield: 720.7 g (fraction 4); 83.6%; >99.5% pure.

7B. Preparation of 1-Benzyloxy-3-chloro-2-(propionyloxy) methoxypropane

204.0 g of 3-benzyloxy-1-chloro-2-propanol, 660 ml of hexane, 626.3 g ofmethoxymethyl propionate and 5.79 g of p-toluenesulfonic acidmono-hydrate were placed into a 2-liter 3-neck round bottom flaskequipped with a thermometer, mechanical stirrer, nitrogen inlet and anefficient distillation column. The reaction mixture was heated toreflux. The distillate was taken off at about 10:1 to 15:1 reflux ratiokeeping the head temperature below 60° C. Samples were taken to monitorthe progress of the reaction. The reaction was quenched when the desiredlevel of methyl acetal was reached which is usually less than about 4%methyl acetal. The quench was carried out by rapidly adding a solutionof 11.55 g of sodium carbonate in 310 ml of water directly to the hotreaction mixture. The aqueous layer was washed with additional waterand/or base to remove any residual propionic acid. The volatiles wereremoved to leave the crude product as a mobile liquid. Yield: 285.2 g,(97.8%); 92.6% pure..

7C. Preparation of1-Benzyloxy-3-propionyloxy-2-(propionyl-oxy)methoxypropane

133.0 g of sodium propionate were placed into a 2-liter 3-neck roundbottom flask equipped with a reflux condenser, nitrogen purge valve,thermometer and mechanical stirrer. 265.0 g of1-benzyloxy-3-chloro-2-(propionyloxy)methoxypropane (from step a) and700 ml of toluene were added, followed by a solution of 23.8 g oftetrabutylphosphonium chloride in 50 ml of toluene. The resultingsuspension was vigorously stirred and heated to reflux for 16 hours.When GC assay of the reaction mixture showed completion of the reaction,the reaction mixture was cooled to 23° C. and 200 ml of 5% sodiumcarbonate solution was added. The mixture was stirred for 15 minutes toallow the salts to dissolve. The aqueous layer is then separated bydecantation. The organic layer was washed twice with 300 ml of water andwas then filtered through a plug (26.5 g) of adsorbent (silica-alumina[Davison Grade 135]). The solvent was removed under vacuum to give 298.2g of product as a pale yellow oil. Yield: 99.4%; assay: 92.5%.

What is claimed is:
 1. A compound of Formula (III)

wherein Y¹ is lower acyloxy, Y² is aralkyloxy, and Z is lower acyloxy orbenzoyloxy.
 2. The compound of claim 1, wherein Y¹ is propionyloxy, Y²is benzyloxy and Z is propionyloxy.